Canadfun Mircralogtst Vol. 16, pp. 75-80(1978)

REFINEMENTOF THE CRYSTALSTRUCTURE OF HVDROBORACITE

CESARE SABELLI AND ANDREA STOPPIONI Istituto di Mineralogia dell'Universitd., Via Iamarmora 4, Firenze, Italy

ABsrRAcr glu (1973); this autlor s0udiedthe changes oc- cu.rring i,n the structure upon loss of water and Three-dimensional counJer-diffractometer X-ray found also that the hardness of the Turkish data aud full-matrix least-squares have been used is 5-6 according to Mohs' scale' whereas to refine the of hydroboracite, et al. (L951) reported its hardness to CaMglBgOn(OH)312.3$zO, wit! a 1L,769(2), b Palache 6.684(2), c 8.235(4)L, p 102.59(2)', Z=2 in space range between 2 and 3. group F2/c. The final R for 1428 reflections is An indexed powder pattern together with the O,042. T\e structure proposed by Rumanova & unit-cell dimensions were given by Cipriani Ashirov (1964) has been confirmed. The basic re- (1958), who also describedthe thermal behavior peat unit of the structure is the [BgOa(OH)g]L group of hydroboracite. Rumanova & Ashirov (1964) consisting of two tetrahedra and one triangle; these gave a correct solution of thti structure on the borate polyanions are polymerized into undulating basis of photographic data. chains along c. Chains of corner-shared Mg(OH)e- (IIzO)a octahedra are linked to boron-oxygen charns giving sheets parallel to (100). Chains of edge- ExPBRTIvIENTAL shared CaOn(OH)n polyhedra also run parallel to the c axis and connect the sheets to each other. The crystals of hydroboracite used in this study come from the Bigadic mine, Balikesir, Sorvrrvrenr Turkey. Indexed X-ray powder data are given in Table 1; indexing was performed, through Irs donn6es tridimensionnelles obtenues aux rayons X sur diffractomltre i compteur ont 6t6 uti- lisfus pour affiner, par la m6thode des moindres

carr6s i matrice entibre, la structure de I'hydrobora- TABLE 1. X-RAI POWDEI DII'FRACItON DAIA FOI SYDROBORACIIET cite, CaMglBgOr(OH)slz.3HO: a 11.769(2), b 6.64(2), c 8.235@)4,, p 102.59(2)' Z - 2, groupe htl d""t"i dobri I h 1 d""1"1 dobrl I spatial P2/c. Le r6sidu final pour 1428 r6flexions 100 r1.486 4 2.L78 2.181 I est 0.042. La structure propos6e par Rumanova & o to 6.69 85 2.L72 2.L72 6 Ashirov (1964) est confirm6e. Dans la structure, Ie 110 5.78 100 1 2.o49 2.088 011 5.14 I -4 2.063 2.063 groupement anionique [83O4(OH)r]'?- est form6 de 4.96 6 2.O57 2.O57 polyanions lt1 4 .464 I 2,O21 2.O27 deux tdtraddres et un triangle; ces sont 210 4.36 13 I polym6ris6s suivant c en chaines ondul6es. Des oo2 4.O18 4.O2 I l:333r.ee6 300 3.829 1.974 r.974 chaines d'octaldres Mg(OHLGIzO)a i sommets com- -202 3.592 3.69 2 -3 L.964 1. 968 3. 589 1.949 muns sont reli6es i des chalnes bore-oxyglne for- r'Yqu 3.56 1 I r.951 mant ainsi des couches (100). Des cha?nes de poly- ot2 1.926 L.926 paralldles 31.0 3.32 74 1.914 1.914 16 bdres CaOa(OH)e i a€tqs communes sont 1. 845 I c et relient les couches entre elles. L L2 3.141 3.L4 -4 1.846 ''644 ' -1 t 1 3. 045 3.05 -6 I t.824 r..827 I (Iraduit par la R6daction) 20 2 3.000 3.00 1.789 1.788 I L2T 2.9 L9 2.921 -L I.770 L.769 2 3ll 2.447 0 L.722 I.722 L 4oo 2.47L 2.873 2 1.715 INrr.ooucrroN 2.838 2.437 I L.7L7 -4 I 1 2.670 2.670 0 1.671 L.67L 7 410 2.637 r.551 L.66L 2 2.624 I 4 1.654 2.530 L z As a part of the systematic investigation of -402 -7 1.650 r'o)r 3 20 2.518 2.517 2 7 the crystal structures of hydrated borate com- o13 2.487 2.486 7 3 o 1.638 2.472 pounds, a refinement of hydroboracite, CaMg- L22 2.436 L4 t .604 500 2.297 7 1.594 1.593 3 fBgOr(OH)sls'3HzO,was performed. Several oc- 222 2.232 2.228 12 t.))) o30 ? .224 2.223 4 r.554 currences of this mineral, commonly with other -5 1 I 2.2L5 1 1. 543 1. 543 1 borates such as colemanite, , inderite and -5 0 2 r.539 1.539 1 130 2.!47 2.L87 L , are described in the literature. Recently, *Pblllp" an occurrence of hydroboracite in Balikesir p*d.r dl.ffrect@ater' co isallatloB, fe fllter, l' 20 Per Dt& province, Turkey, was described by Demircio- range 5" 5 20 < 70". 75 76 THE CANADIAN MINERALOGIST

TA3LE 2, CRYSTALLOCT.APNlC DATA AND OTIIDR INIORYAIlON trolled diffractometer using MoKa ra.driationand the a-20 scantechnique. A total of 1846 indepen- dent reflections in the range 3o<20(30o Folou1a unit, caMg [8304(oF)31 were r.:uro measured;a reflection was consideredto be ob- a - 11.769(2).& crvstal size: o.35xo.o9xo.05 (@) served if its intensity was greater than five stand- - b 6.684(2) i ra alfittert rro/zt ard deviations based on counting statistics. Ap- c - 3.235(4) i plication roral no. of lrol : 1846 of this procedure gave 1428 observed p - 102.59(2). reflections. Intensities were conected No. of lr.ol> 5s : 1428 for I-e. rentz-polaization effects, rrhereas absorption Spaee group P2/c Z - 2 correction was considered negligible becauseof V - 632.3 i3 p(UoKa) : 6.3 cr-1 the crystal's thinness. D obs - 2.15 gcn-3 tinaL R (ob6erved):O.042

D calc- 2.170 gcn-3 Fioal F (a11 data): O.060 STRUcrr.rRERnrtNnptBivr

o -f1lj rol _ Incllvflrol The refinement of the structure was started Tenpe!atu!e fdctor fore used: using the atomic coordinates of Rumanova & u*p{-(Frrh2r prrt2+z Ashirov (1964) and giving P,ru2+ 3rr*rz Plrlrr: 82rr.DI each atom a I value obtained by averaging the B's found by these authors for different layer-lines of Weissenberg repeated cycles of cell-constant refinements, data. with tle aid of single-crystalintensities. The re- One least-squaresfull-matrix refinement oycle fined parameters,based on 6L lines, are reported with isotropic thermal parameters and two sub- in Table 2. T\e observeddensity, determined by sequent full.matrix cycles with anisotropic ther- the flotation method, is 2.159 cm{ and the calcu- mal parametersled to a residual index R:0.M8. lated value, 2,l70g cm{. Intensitieswere collected At this stage of refinement a difference Fourier with a Philips PW 1100 4-circle computer-con- synthesiswas computed in an attempt to direct- iy locate tlte six hydrogen atoms of the structure. Since the O-O distance analysis showed an un- ambiguous hydrogen-bonding system, tle resi-

TABLE 3. TRACTIONA', ATOMIC COORDINATDSAND ISOTROPIC TEMPERATURET'ACTORS

Atoe n ti2r

!tg 0 0 0 0.75 ca 1"/2 0.47388(9) L/4 O.52 ** o (l) 0 0.1238(3) L/4 1.01 0(2)** 0.0504(2) o.2772(3) 0.5966(2) 1.37 N: o(3). 0.1667(1) 0.906r(2) 0.5359(2) o.99 0(4) 0.1910(1) o.3419(2) o.2468(2) 0.80 0(5) 0.1758(l) o.5515(2) o.4678(2) 0.89 0(6) 0.3487 (1) o.7299(2) o.6052(2) 0.80 0(7) 0.3266(1) 0.6114(2) o.3209(2) 0.87 0 (8)* 0.3544(1) 0.9672(2) 0.3936(2) 1.16 0(9)* 0.5093(1) O.7160(2) O.47t7(2) O,97 3(1) o.2251(2' O.7125(4) 0.59t2(3) O.Ot B(2) 0.3848(2) 0.7614(3) 0.4480(3) o,69 B(3) o.2325(2) 0.5039(3) 0.3436(3) o.63 H(1) 0.064 0.197 0.249 4,5 H(2) o.o92 0.370 0.554 4.5 l1(3) -0.025 0.335 0.575 4.5 E(4) o .230 0 .927 0.489 4.5 B(5) 1.069 o.298 Frc. 1. Projection of the structure down the D axis. H(6) o.446 o.L79 0.483 Half-cell rn ,r is represented. Open circles and small black circles indicate oxygen and hydrogen Bt€ ol the non-hydrogeo atoms a!e the equivalent oires atoms, respectively. after HaEilton(1959). *O to 0Et rro 10 rater Dolecule€ STRUCTURE OF }TYDROBORACITB 77

IABIB 6. ANISOTROPICTEWESATURE TAC1OR COTTTICISNTS* TABLE 5. SELECTEDIN1EPATOYIC DISTINCES

AtoE ? tt Pt, F., Pr" P,t Pn Ng octahedlon B(1) tetrahcdlon us-o(1),o(1iv) z.zrg(r) i B(1)-o(3) 1.491(5) I (g) Ms 123(1) 363(22) 383(15) -26(10) 62 (8) -1.6(X4) us-o(2vi),o(2ix) 2.554(5) s(r)-o(ctit) r.aeg Me-o(3""")'0(3-) 2.o2o(6) B(r)-o(5) 1.508(8) ca 78(4) 359 (r3) 224 (9) o 64(5) 0 B(1)-o16) 1.440(4) Uean 2.098 o(1) r75(15) 605(47) 439(34) o 121(19) 0 YeaD 1.411 Ca Dolvhedron o(2, 224(13') 619(34) 750(29) 49(76') 152(15) 80 (25) !*f,fitflo."tii; z.:rr{a) o(3)-o(4'"') 2.374(\) (32) (26) -75 (15) -2LL(22) 0(3) 154(11) 584 482 132(L3) ea-o(7),o(7v) 2.425(4) o (3)-o (5) 2.446(2) 2.4o4(9) 0(4) 163(11) 5L3 (32) 297 123' -7 4 (L4) 131(r3) -48 (20) ca-o(9),o(9v) 2.424(3) o(3)-o(6) cr-o(gii),0(g"iii1 z.ea:{:) o(auii)-o(:) 2.374<4) 0 (5) 149(r1) 603(32) 35r(23) 69(15) rr6(15) 134(21) o(r,"ti)-o(o) 2.446(9) UeaD 2.461 o (6) r35 (1r) 54r (31) 298(22) -19 (14) ?i/1r\ r

Uean 2.374 0(6)-o(7) 2.430(6) *a11 5. o(6)-o(8) 2.469(3) valuee r 10 B-B dig!ancea o(6)-o(9) 2.386(9) E(1)-B(3) 2.519(4) dual peaks attributable to the six hydrogen atoms o(7)-o(8) 2.458(2) B(1)-B(2) 2.447(9) Unfortunately 0(7)-o(s) 2.342(9) were recognized on this map. B(1)-B(3) 2.'498(9') these maxima rilere rather diffuse and therefore o(8)-o(9) 2.460(9) did not give sufficiently reliable indications for Ueatr 2.424 an accurate positioning of the H atoms; it was SyoEetry coda: considered prefemble to locate H atoms at cal- vi) -x t l'z culated positions ls of the distance from the lotre a J i) l-x 2-Y vii) : 1-J l+z -lrz donor oxygen toward tle acceptor oxygen along ii) 1-! l-Y L-z viii) : l-Y -, -l +" the hydrogen bridge.s previously recognized on iii) -a l-y ix) x -v :) -x -l+Y l-z tle basis of o-o distances. Inclusion of these iv) . N 2-9 'L+z atoms in the structure factor calculations with v) l-x t t-z xt) rough coordinates and B=4.5k reduced R to 0.M2 for observed reflections and to 0.060 for

.. ,

-: 78 THE CANADIAN MINERALOGIST

totnp Fro.3.-A c-axis projection of the structure. Dotted lines indicate hydrogen bonds. Small numbers label the hydrogen atoms (small black iircles); larger numbers designatethe oxygens(open circles) belonging to two water moleculesand to the three hydroxyl groups.

all data. Becauseof some inconsistenciesin thE the Depository of Unpublished Data, CISTI, shifts, the hydrogen atom parameters were not National Research Council of Canada, Ottawa, refined. Ontario KlA 0S2. The structure refinement was performed with the CII 10070 computer using a modified ver- DrscussroN sion of the full-matrix least-squares Drosram ORFLS @using er a/. 1962). Neutial-itom Figures l, 2 and 3 show the atomic arrange- scattering factors for Mg, Ca, O and B were ment in the structure of hydroboracite. Inter- taken from Cromer & Waber (1965), while for atomic distancesand angles are given in Tables H the values by Stewart et al. (1965) were used. 5 and 6 respectively. The positional and isotropic thermal parameters The Mg atoms lie on symmetry centers and are given in Table 3; anisotropic tlerrnal para- are octahedrally coordi,nated by four water meters appear in Table 4. A table of structure molecules (O(1), O(2) and their sy.mmetryequi- factors may be obtained at nominal charge from valents) and two hydroxyls (O(3) and its sym-

TABLE 6. SELECTED IITEBATOTIC ANCL!s

Ca polyh6dro! Xa ocrahodron 8(2) totlahadrol - o(6"^')-ca-0(6") r09.1(r)' o(l) {s-o(2il) 88.0(1). 0(7)-B(2)-o(8) rll.8(2). o(7) ?6.0(r) o(3.'") 86.8(l) 0(9) r04.r(2) o(r") r33.1(l) o(:t') 9?.0(r) 0 (6) rro.2 (2) o(e) 132.4(r) o(3-) 93,2(t) 0(8)-8(2)-o(9) 113.8(2) o(gt) 95.8(r) o(ztl)-ug-o(lotlt) 88.0(r) o(6) 108.0(2) o(sll) s9.:(r) o(l^') 92.0(l) o(9)-B(2)-O(6) tO9.o(2) o(9'...) 56.7(r) o(3x) 92,0(rt o(7) - ca-o(6tl) 133.r(r) o(2") rlJ.4 (r) B(l) t6!r!h6d!o! l(3) trispsle 0(9) 57.8(r) o(:) - r(r)-o(a"tt) roo.o(z). o(4)-8(3r-o(7) r22.2(2). o(9") 91.5(l) 0(5) r09.3(2) 0(5) 116.5(2) o(stt) 77.1(1) o(5) llo.2(2) o(:)-8(3)-O(5) r2L.3(2t 0(9.-'^) r:r.9(l) ora'lI)-r(r)-o(s) r05.6.(2) (9) - 0 ca-o (9v) 96.2(r.t o(6) l14.J(2) o(gtl) 70.8(r) o(5) - 8(1)-0(6) 110.2(2) o(9"'.) r65.9(r) STRUCTT'RE OF IIYDROBORACITE 79 fiietry Qquivalent).These Me(H'O)r(OE)z octa- trahedra. In.hydroboracitg indeed, this distance hedra are linked to each other through O(1) is indicative of a hyd.rogenbond, with O(3) as corners into chains that extend along c. An the donor and O(8) as the acceptor oxygens interesting feature is the- greater length of the (Figs. 1 a,nd 3). The interatomic distances and MF-O(l) distance Q.2l9A) in the c direction in angles found in the present refinement agtee relation to the Mg:-O distancesperpendicular to well with those found for other similar borate the chain (2.020 and 2.054A,). The mean value compounds. of 2.098A is close to the values found for octa- As Figures L and 2 show, each B-O chain in hedrally coordinated magnesiurn in other struc- hydroboracite run$ parallel to c and undulates tures. The coordination polyhedron is regular, in the D direction. Each chain is joined through the greatest deviation from 9Oo being 3.2'. the hydroxyl O(3) to the Mg chain at }l=0 and The Mg-Mg distancein the chains is 4.12A. the adjacgnt Mg chain at y=l in turn. As Figure The Ca atom lies on a twofold axis and is 8- 2 illustrates. the unit cell contains two B-O coordinated by four hydroxyls and four oxygen chains, placed one upon tle other, connectedto atoms. The Ca-O distancesrange from 2.351 to the same Mg chain; thus each Mg chain, having 2.64A; the averageis2.46lA, slightly lessthan the Mg octahedra on sym.metrycenters, is linked the sum of effective ionic radii given by Shan- to two B-O chains on one side and two more non (1976). The Ca polyhedron was well de- symmetrical B-O chains on the other side, scribed by Rumanova & Ashirov (196$; in shape making up O-B-O-Mg-O-B-O sheets in the bc it is halfway between a right orthorhombic prism plane. Each Ca chain, which also extends along and an orthorhombic. antiprism. Each Ca poly- c approximately at Vzy (Fig. 1), provides tle hedron lies on the two-fold axis and shares its connectionsbetween the sheetsbridging two ad- hydroxyls (O(9) and its sy.m.metry equivalents) jacent B-O chains belonging to different sheets. with adjacent Ca polyhedra to form a chain of Further connections are also provided by the edge-shared polyhed.ra running parallel to the hydrogen-bond system in the following way Mg chains and at Yzx, 1/zy from them. The (see Fig. 3): the hydrogens H(1) and its sym- C*Ia" distance is 4.13A. metry equivalent of the first water molecule and The strusture of hydroboracite contains the H(2) and H(3) of the secondwater molecule as [BaO4(OH)a]'-polyanion consisting of two boron wel,l as H(4) and H(5) belonging to two hydroxyl -oxygen tetrahedra and a boron-oxygen triangle, gtoups make intra-sheet connectionso whereas corner-linked in such a way as to build up a H(6), belonging to the tltird hydroxyl, is the boron-orygen ring. These groups are linked to only inter-sheet hydrogen bond. each other to form infinite boron-oxygen chains. The structure is in accordance with the ob- The structural unit, consisting of a pair of te- served (100) and (010) cleavagesof hydrobora- trahedra and one triangle (21, A), is a very com- cite: the first cleavage involves breaking of mon R-O group found in polyborate structures O(9FH(6) . .. O(8) hydrogen bonds and of four and is evidently a particularly stable configura- out of the eight Ca-O bonds; the (010) cleavage tion. Examining the mutual arrangement of the breaks Mg-O(3) bonds and three of the remain- boron tetrahedra in these rings, Rumanova ing H bonds. (1972) stated that the boron-oxygen radicals be- A water geometry test using the Ferraris & long to two forms defined on the basis of the Franchini-Angela (1972) scheme was carried respective orientations of the two tetrahedra in out. I.n the first water molecule the oxygen O(1), the ring, either both oriented in the same direc- which lies on the two-fold axis. coordinatestwice tion or alternating in direction. This fact can lead to a better description,of borate compounds ?ABLE 7. 1I'DROCEN-BONDINCSYSTEI{ that contain this elementary unit either as an isolated po{yanion or in polymerized for,ms. Donor

The radical found in hydroboracite and the one atoD (D) g aloF(A) D,,,A present in colemanite, which shows the same charasteristic chains (Christ er boron-oxygen o(1) n(1) o(4) z.oaoeli o.gni t.tsit rr4-2rr)c in ) al, t958), are close to tLe first limiting form o(r) H(r'i).0(aui) " Rumanova's scheme. As seen especially in Fig- o(2) H(2) o(5) 2.841(9) o.91 1.81 , u.le Figures.2 and 3, both tetrahe- 102.8(1) L but also in ',l dra ar.e oriented in the same direction; more o(2) H(3) o(siii) 2.112(g) 0.9 r 1.89 than thal the distance (A.7494) between the 0 (3) n (4) 0(8) 2.1t9(9) 0.92 1.83 corners O(3) and O(8) of the two tetrahedra is 0(8) n(s) o(a"i) 3.r.12(3) 1.03 2.07 very short compared with the same distance rn o(si) H(6) 0 (8) 2.743 (9) 0.91 1..83 other cornpounds with this arrangement of te- 80 THE CANADIAN MINERALOGIST

TAB'E 8. CSATCEDA1ANCE RsFSRENcEs

BRowN, I. D. & SrreNNory R. D. (1973): Empirical Atoa ca He B(1) B(2) B(3) u- ...H Sus9 bond-strengtl-bond-length curves for oxid"". Acta Cryst. A2g, 266-252. 0(1) O.2tr^" 0.78 1.02 Btlgtg.W.R., MannN, K. O-._&LBw, H. A. (1,962): 0(2) o.3l^2 l..61 1.98 oRFLs, a Forrran lrystallographil ieaJi-iqiii"s 0 (3) o.39"2 o,r2 o.80 program. U.S. Nat. Tech. Inf. Serv. ORLN_i05. o (4) o.76 1.00 o.22 1.98 CHRrsr, C. L., CLARK, J. R. & EveNs, H. T. Jn_ (1958): 0(5) 0.70 1.00 o.39 2.09 Studies of borate (III): The crystal structure of colemanite, CaBrOn(OH)s.HrO.. 0(6) o .32"2 a .82 o .1 7 2.O4 Acta Cryst. LL, 761-770. 0(7) o .26^" 0.70 1.o0 1.96 CnruANr, C. (1958)l Ricerche sulla disidratazione 0 (8) o.77 0.47 0.40 2.O4 di alcuni borati naturali. Atti Soc, Tosc. Sci. Nat. 0(9) o.42-- 0.16 0.80 1.98 4.65,284-322. Ch.oMER,D. T. & Wans& J. T. (1965): Scattering factors computed Sond atlelgths of fi16t lide are to be doubled becsuse from relatirvistic Dirac-Slater wave functions. Acta Cryst. 18, 104_109. 0 (1) is ia epecial poeitioa. DEldrRcrocLU, A. (1973): Boron minerals of Tur_ key: hydroboracite. Bull. Min. Res. Expl. Inst. Turkey 80, lO+1L7, the Mg cation along the two lone-pair orbitals DoNNAy, G. & DorNel J. D. H. (1973): Bond- with angles€g {rod €gof 42o arld 43o, respective- valence summation for borates. Acta Cryst. B2g, ly; this water molecule therefore belongs to 14t7-t42s. class2, type B. The secondwater molecule, with FeRnerus, G. & FneNquNr-ANcrLA, M, (1972): the oxygen O(2) in general position and co- Survey of the geometry and environment of wa- or{inating o,nly one Mg cation along a lone-pair ter molecules in crystalline hydrates studied by orbital, has a er angle of 37o and can be included reutron diffraction. Acta Cryst. BZB, 3572-3593. in the class L', type J. In Table 7 the hydrogen- HArflLToN, W. C. (1959): On the isotropic temper- bonding system and the donor-acceptor, donor- ature factor equivalent to a given anisotropic hydrogen and hydrogen-acceptor distances are temperature factor. Acta Cryst. L2, 609-610. reported together with the two water angles. Par.ecne, C., BERMAN,H. & FnoNDH., C. (1951): An electrostaticvalence balance was computed lhg System of Mineralogy 2, 7th ed,, John Wiley according to the method given by Brown & & Sons. New York. Shannon (1973). For the H bonds the curve bv RulrANovA, I. M. (1972): Forms of the ternary those authors quoted in Donnay & Donnay [BrOr]t- boron-oxygen ring in the structures of hydrated (1973) was employed. Table 8 shows the borates. Soviet Phys. Cry.r. 16, l0lg- con- 1020. tributions of different atoms and the bond- strength sums (v.u.). As can be seen,the valence & Asnrnov, A. (1964): Determination of the crystal structure of sum for O(1) is slightly positive in spite of the hydroboracite CaMg[BsOa- (OH)blr.3HrO. Soviet Phys. Cryst. 8, 665-680. fact that the shortening of its hydrogen bond is SHANNoN,R. D. (1976): balanced with the lengthening of the Mg-O(l) Revised effective ionic radii bond. and systematic studies of interatomic distances in halides and chalcogenides. Acta Cryst. A32, 757-767. Sruwanr, ACKNoWLEDGMENTs R. F., DavnsoN, E. R. & SrrwpsoN, W. T. (1965): Coherent X-ray scattering for the hy- ..Consiglio drogen atom in the hydrogen molecule. t, Chem. This work has been supported by Phys. 42,3175-3187. Nazionale delle Ricerche, Cenho di studio per la Mineralogia e la Geochimica dei sedimenii',. Manuscript received August 1977, emended Novem- Firenze, Italy. ber 1977.